1. Field of the Invention
This invention relates to a method for the prevention of drooling from a sprue gate of injection molding mold and nozzles of injection apparatus.
2. Description of the Prior Art
When nozzles are moved away from the mold after injection molding has been carried out, trickles of plastic from the sprue gate occur. This trickle of plastic is generally called the drooling (trickling), which is said to be caused by residual pressure stored in the sprue and runner.
The drooling on the side of mold tends to occur when the hot runner is used to inject fused plastic into each of cavities, and for this reason, a needle valve is mounted on each nozzle connected to the hot runner. As a result, the drooling in each nozzle can be prevented but conversely the drooling in the sprue gate increases, as a consequence of which the trickled plastic sticks and/or burns to a peripheral portion of gate or to the nozzle tip of injection apparatus, resulting in a poor nozzle touch.
Similarly, even in the nozzles of injection apparatus, when the nozzles are moved away from the sprue gate after injection molding has been carried out, the drooling from the nozzle orifices occurs. This is said to be caused by residual pressure of fused plastic stored between the nozzle and the screw.
Means for preventing such drooling have been heretofore proposed, one of which is that a needle valve is provided within the nozzle, and the other being that a screw is forcibly withdrawn slightly to zero plastic pressure. However, in the former in which the needle valve is biased by a spring, the drooling may be prevented even when back pressure is applied, but high plastic pressure is required to open the valve during the process of injection, thus entailing a considerable resistance. As a consequence, it is not possible to make delicate adjustment during injection and filling processes important to molding and in addition there gives rise to new drawback that one gets burnt easily.
In the latter proposal in which the screw is withdrawn, if the forcible withdrawing stroke of screw is too long, the injection cylinder interiorly assumes a negative pressure so that the nozzle orifice draws air, which during next injection, is injected together with fused plastic into the mold, resulting in defective molded articles. Accordingly, the forcible withdrawing stroke of screw must be made accurately, but it is said to be extremely difficult to accurately control the withdrawal of screw within the range of several millimeters.